The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuit structures is well known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Significantly improved substrates for the fabrication of such circuits are disclosed and claimed in Hang et al., U.S. Pat. No. 4,256,796, issued Mar. 17, 1981, the disclosure of which is incorporated herein by reference. The Hang et al. substrates are metal coated with an improved porcelain composition comprised of a mixture, based on its oxide content, of magnesium oxide (MgO) or mixtures of magnesium oxide and certain other oxides, barium oxide (BaO), boron trioxide (B.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2). The preferred metal is steel, particularly low carbon steel, which may be coated with various other metals such as, for example, copper. The porcelain compositions are applied to the metal core and fired to provide a partially devitrified porcelain coating on the metal core. The coating has a very low viscosity at its initial fusion point and then almost instantaneously obtains a high viscosity due to devitrification. The fired coatings, which are preferred for hybrid circuit applications, have a deformation temperature of at least 700.degree. C. and a high coefficient of thermal expansion of at least about 110.times.10.sup.-7 /.degree.C.
While the porcelain metal substrates of Hang et al. represent a significant improvement over previously known substrate materials, they are disadvantageous only in being incompatible or poorly compatible with commercially available thick-film inks. With the development by us of novel inks compatible with the Hang et al. boards, there remained the need for an improved overglaze composition which would be compatible with the Hang et al. boards and with inks formulated therefor. Overglaze inks are utilized to provide mechanical and environmental protection for the portion of films of various functions predominately resistor and conductor films which is not otherwise covered, e.g., by components. Such an improved overglaze ink is provided in accordance with this invention.